Sweep out some extra warnings

Swept:
`-Wpadded` in some places (where it caused extra size, instead of just
saying "hey, we padded struct at the end to align, just as standard says")
`-Wweak-vtables` everywhere (Clang)
`-Wexit-time-destructors` everywhere (Clang)
`-Wmissing-noreturn` everywhere (Clang)

The last three are enabled for Clang compilation going forward.

Also enabled `-Wunreachable-code` for Clang and GCC

include/internal/catch_message.h

@@ -21,9 +21,9 @@ namespace Catch {
                         ResultWas::OfType _type );
 
         std::string macroName;
+        std::string message;
         SourceLineInfo lineInfo;
         ResultWas::OfType type;
-        std::string message;
         unsigned int sequence;
 
         bool operator == ( MessageInfo const& other ) const;